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PARIS — The gun fired four shots into a gelatin block. Each nine-millimeter bullet punched deep into the substance, which was meant to mimic the density of a human body.

For the experts at the Austrian Interior Ministry performing the test, it was a clear sign: This was a deadly weapon.

But it was no ordinary gun. The officials had downloaded the gun’s digital blueprints from the Internet and “printed” the weapon on a type of 3-D printer that any person could buy online for about $1,360. It took the Austrian authorities 30 hours, and maybe $68 worth of plastic polymer, built up layer by layer according to the software instructions, to make the gun.

“Our interest was to see if the manufacturing of a working gun using this technology is possible,” said Karl-Heinz Grundböck, a spokesman for the Austrian Interior Ministry, which performed the test in May. “The answer was a very clear ‘Yes.’ ”

Law enforcement agencies across Europe are on alert over the proliferation of gun-making software that is easily found on the Internet and can be used to make a weapon on a consumer-grade 3-D printer. So far, there are no reported episodes of violence committed with such weapons, but police officials worry it is just a matter of time.

In May, after a 25-year-old law student from Texas named Cody Wilson posted designs for a 3-D-printed handgun online, the files were downloaded more than 100,000 times in just two days before the State Department demanded they be removed. Spain led the ranking of downloads at the time, followed by the United States, Brazil, Germany and Britain. A full version of the gun, called the Liberator, went on display last month in the Victoria and Albert Museum in London.

No wonder that in the European Union, which has much stricter gun-control laws than the United States, officials worry that it is becoming much easier to covertly obtain and carry potentially lethal weapons.

“In Germany and in most European countries, the possession of an unregistered weapon, even if it is manufactured at home, is illegal and punishable by law,” said Michael Brzoska, a security expert and director of the Institute for Peace Research and Security Studies at the University of Hamburg. “But the temptation to try, if it’s technically possible, is a great one.”

Despite the State Department’s attempt to block them, the printing instructions for Mr. Cody’s Liberator have continued to spread and are available for free download on sites like the Pirate Bay, a popular file-sharing portal.

Stoking the anxiety have been well-publicized examples in recent months of people evading airport-style security scanners with 3-D-printed plastic weapons, whose only metal components are firing pins no bigger than a short common nail. Two reporters for the British newspaper The Mail in May smuggled such a gun onto a packed Eurostar train from London to Paris. And last summer, a reporter from Channel 10 television station in Israel successfully toted a 3-D-made handgun into the Israeli parliament, the Knesset, where Prime Minister Benjamin Netanyahu was giving an address.

“The development of 3-D-printed weapons is still in its infancy,” said Troels Oerting Joergensen, head of the European Cybercrime Centre at Europol, the European Union’s law enforcement agency. “But such guns can fire a bullet and they can probably kill. It is a very unwelcome development.”

The gun designs are evidently getting better by the month. Although early versions of the Liberator could be fired only a few times before the barrel needed replacing, a YouTubevideo emerged in August that apparently shows a 3-D-printed rifle called the Grizzly 2.0 successfully firing 10 shots.

The manufacture of weapons using 3-D printers is already banned by a European Union directive to member nations. Enforcing that rule, however, may prove a challenge.

Following the example of their Austrian colleagues, German police officials are testing the technology themselves. Europol has recently purchased a 3-D printer to manufacture its own weapon. Authorities in Spain, Switzerland, the Netherlands and Britain said they, too, were monitoring the developments of the 3-D printing technology.

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